Throttling valves or control valves are known from the prior art. These valves may be used for controlling the flux of a fluid stream and possibly also for enlarging liquid droplet sizes in the fluid stream flowing through a throttling valve. The term throttling valve is used to denote valves to control either one or more process parameters such as, but not limited to, flow, pressure, temperature, liquid level etc.
In the oil and gas industry control valves are used to control pressure, level, temperature and flow. In some cases these control valves operate at choked conditions, once sufficient pressure drop is created over the control valve. In processing natural gas this pressure reduction over a valve causes the temperature to drop without extracting heat or work from the gas (i.e. adiabatic). This so called throttling process is isenthalpic by nature since part of the potential energy which was available for thermodynamic work, has been dissipated inside the valve. For real gas conditions—such as high pressure natural gas—intermolecular forces are dominated by association forces, hence said isenthalpic expansion process results in what is known as Joule-Thompson (JT) cooling. The resulting temperature decrease is merely due to the decrease of the internal molecular energy whilst the enthalpy remained unchanged. The valve creating this pressure reduction is called a JT valve. The cooling effect over a JT valve may be used to condense a part of the natural gas stream, such that a liquefied and/or solidified fraction can be separated in a vessel. For the majority of these separator vessels the driving force is either inertia or gravity forces or in other words the masses of the liquefied drops determine the efficiency of the separation. Such a Low Temperature Separator preceded by a JT valve is normally referred to as a JT-LTS system.
For instance, a traditional cage-valve is known for control service as supplied by Mokveld Valves B.V. in which the flux of fluid is throttled over a perforated sleeve 23. A piston-type valve body 22 may be provided in the perforated sleeve 23 to control the flux through the perforated sleeve 23. This cage-valve is described in more detail below with reference to FIG. 1a-d. 
The conventional Mokveld throttling valve shown in FIG. 1a comprises a valve housing 21 in which a piston-type valve body 22 is slideably (see arrow 8) arranged in the associated perforated sleeve 23 such that by rotation of a gear wheel 24 at a valve shaft 25 a teethed piston rod 26 pushes the piston type valve body up and down into a fluid outlet channel 27 as illustrated by arrow 28. The valve has a fluid inlet channel 29 which has an annular downstream section 29A that may surround the valve body 22 and/or perforated sleeve 23 and the flux of fluid which is permitted to flow from the fluid inlet channel 29 into the fluid outlet channel 27 is controlled by the axial position of the piston-type valve body 22 relative to the associated perforated sleeve 23.
The conventional sleeve 23 comprises openings 30—perforations, slots or holes—that have a radial orientation i.e. rectangular to the cylindrical surface of the sleeve 23. This is shown in FIG. 1b, being a cross-sectional view of the perforated sleeve 23.
By displacing the valve body 22 in the sleeve 23 in axial direction the flow area can be controlled.
As illustrated in FIG. 1c the flow pattern in a cage valve 23 with radial openings is highly disordered, hence introducing high shear forces causing droplets to break up into smaller droplets. FIG. 1d schematically illustrates the uniform mist flow with small liquid droplets in the fluid outlet channel 27, and illustrates that the concentration of droplets in the fluid outlet channel 27 is substantially uniform (indicated by the uniform grey shading).
Even though the prime function of a JT valve is flow rate control, it is often forgotten that the second function is to create a separable liquid phase. In the gas processing industry the mean droplet size resulting from an isenthalpic expansion over a JT valve is unknown, hence the separation efficiency of downstream separators is to a large extent unknown. From time to time gas quality problems do occur due to suboptimal separation efficiency. In those cases it is often the hydrocarbon dew point, which remains too high, which indicates that especially hydrocarbon droplets tend to be too small.
WO2006070020 describes an improved valve, that increases the separation efficiency. This will be discussed in more detail below with reference to FIG.'s 2a-2d. 
The valve shown in FIG. 2a comprises a valve housing 21 in which a piston-type valve body 22 is slideably (see arrow 8) arranged in the associated perforated sleeve or cage 123 such that by rotation of a gear wheel 24 at a valve shaft 25 a teethed piston rod 26 pushes the piston type valve body up and down into a fluid outlet channel 27 as illustrated by arrow 28. The valve has an fluid inlet channel 29 which has an annular downstream section 29A that may surround the valve body 22 and/or perforated sleeve 123 and the flux of fluid which is permitted to flow from the fluid inlet channel 29 into the fluid outlet channel 27 is controlled by the axial position of the piston-type valve body 22 relative to the associated perforated sleeve 123. The valve may furthermore comprise a conical central body 15 which is substantially co-axial to a central axis 11 of the fluid outlet channel 27 and which generates an outlet channel 27 having a gradually increasing cross-sectional area in downstream direction, thereby generating a controlled deceleration of the fluid flux in the outlet channel 27 and constituting a vortex that promotes growth and coalescence of condensed fluid droplets or bubbles in oil.
FIG. 2b illustrates that in the throttling valve the perforated sleeve 123 comprises tilted or non-radial openings 130, that are drilled in a selected partially tangential orientation relative to a central axis of the perforated sleeve 123 such that the longitudinal axis 12 of each of the openings 130 crosses the central axis 11 at a distance D, which is between 0.2 and 1, preferably between 0.5 and 0.99 times the internal radius R of the sleeve 123.
The tilted openings 130 create a swirling flow in the fluid stream flowing through the fluid outlet channel 27 as illustrated by arrow 14. The swirling motion may also be imposed by a specific geometry of the valve trim and/or valve stem and/or valve housing. In the valve according to FIG.'s 2a and 2b the available free pressure is used for adiabatic expansion to create a swirling flow in the fluid stream. Since no thermodynamic work is exerted on, or delivered by the expanding fluid with respect to its surroundings, said adiabatic expansion can be considered as an isenthalpic process. The kinetic energy is mainly dissipated through dampening of the vortex along an extended pipe length downstream the valve.
As illustrated in FIG. 2c the flow pattern in a cage valve with tangential openings is ordered and has a swirling motion, hence reducing shear forces which can cause droplets to break up into smaller droplets and promotes coalescence of micro droplets/bubbles. FIG. 2d schematically illustrates the mist flow with small liquid droplets concentrated in the outer perimeter of fluid outlet channel 27.
As illustrated in FIG. 2d the presence of a swirling motion in the throttling valve concentrates the droplets 18 in a reduced flow area 7A at the outer boundary (about 60% of total cross sectional area) of the fluid outlet channel 27 (higher concentration indicated by darker shading), such that the droplet number density increases with a factor of circa 1.7. Furthermore the rate of turbulent dissipation in de vortex core is large because of the high tangential velocity.
It will be understood that the creation of large liquid droplets (or large gas bubbles in case of oil or condensate degassing) in the outlet channel 27 of the throttling valve will make it easier to separate the liquid and gaseous phase in a fluid separation assembly that may be arranged downstream of the throttling valve. Such a subsequent fluid separation assembly may comprise one or more gravity and/or cyclonic separation vessels.
The fluid could be either 1) a pre-dominantly gaseous carrier with a liquid phase or 2) a predominantly liquid carrier with an immiscible liquid and/or gaseous phase. An example of option 1) is a LTS process with a JT-valve fed by a natural gas stream with liquid fraction of condensates, water and glycol. An example of option 2) is an oil or hydrocarbon condensate stabilization process with a throttling valve fed by an oil or condensate stream with liquid fraction of water and/or glycol and entrained gas.
FIG.'s 2c and 2d illustrate that the advantage of creating a swirling flow in the outlet channel of the valve is twofold:                1. Regular velocity pattern−>less interfacial shear−>less droplet/bubble break-up−>larger drops, and        2. Concentration of droplets in the outer circumference 7A of the flow area of the fluid outlet channel 7 or concentration of droplets in the centre of fluid outlet channel 7−>large number density−>improved coalescence−>larger drops/bubbles 18.        
Solidification
By cooling a fluid stream in a process (e.g. expansion cooling, refrigeration cooling etc) the condensed fraction may (partially) solidify to for instance crystalline solids. For well fluids produced from a subterranean reservoir, these solids may comprise gas hydrates, oil waxes, asphaltenes, resins, carbon dioxide, hydrogen sulphide etc.
Gas clathrate, also called gas hydrate or gas ice, is a solid form of water that contains a large amount of gas molecules within its crystal structure. Such gas clathrates are found in formation fluids e.g. oil or natural gas, where some of the gas components (e.g. methane, ethane, propane, (iso)butane, carbon dioxide, hydrogen sulphide) can form hydrates in conjunction with water at elevated pressure. These hydrates usually exist in agglomerated solid forms that are essentially insoluble in the fluid itself.
Thermodynamic conditions favouring gas hydrate formation are often found in pipelines, transfer lines or other conduits, valves and/or safety devices, vessels, heat exchangers etc. This is highly undesirable because the gas crystals might agglomerate and cause plugging or blockage of the flow-line, valves and instrumentation. This results in shutdown, loss of production, risk of explosion and injury or unintended release of hydrocarbons into the environment either on-land or off-shore. Accordingly, natural gas hydrates are of substantial interest as well as a concern to many industries, particularly the petroleum and natural gas industries.
Carbon dioxide (CO2) crystals may form when cooling a CO2 containing well fluid to temperatures below −60° C. Processes intentionally processing fluids to produce CO2 solids are known from WO9901706 and WO03062725.
Waxes, resins, asphaltenes may form in a well fluid containing oil which is cooled for instance in a pressure let down (i.e flash) vessel.
Accordingly, the throttling valves as described above with reference to FIG.'s 1a-2d are prone to such problems. During use, the (tilted) openings 30, 130 may get (partially) blocked by solids comprised in the fluid stream. Said solids may then tend to stick to the interior of the valve, such as to the entrance and inside of the (tilted) perforations 30, 130, thereby partially or completely blocking the (tilted) openings 30, 130.
Short Description
It is an object to provide a throttling valve that overcomes at least one of the above identified problems of openings getting obstructed by solids, such as hydrates.
According to an embodiment, there is provided a throttling valve comprising a fluid inlet and a fluid outlet, the throttling valve being arranged to control a flux of a fluid stream flowing via a flow path from the fluid inlet to the fluid outlet, the flow path comprising a plurality of openings which, in use, create a pressure reduction over the throttling valve and thereby a cooling effect of the fluid, wherein the openings widen in a downstream direction. The openings may have a divergent angle φ in the range 10°-50°. Also, the openings may have a radial, tangential or axial orientation or direction with respect to a central axis. The openings may also have a combination of tangential and axial orientation or direction with respect to a central axis.
Such a throttling valve has the advantages that the openings will have less chance of getting blocked by solids, due to the tapered shape of the openings.